Particular types of power supplies may convert the alternating current (ac) line delivered from a wall socket to the direct current (dc) power usable with many of today's electronics. A rectification circuit, such as a full rectifier and PFC circuit, may be used to convert the ac voltage to a dc output voltage. A dc-dc power converter included in the power supply may then be used to convert the dc output voltage from the rectifier circuit to a regulated dc voltage for use in electronic devices.
In one example the dc-dc power converter uses a power conversion topology known as a forward power converter. A forward power converter may use either one or two active switches to apply an input voltage to the primary winding of a transformer. In the forward converter, a secondary winding on the transformer produces a ratio of the voltage on the primary winding based on a turns ratio between the primary and secondary windings. The voltage on the secondary winding is then rectified and filtered to become an output voltage.
In the forward power converter, the output voltage is normally regulated by a control circuit. The control circuit compares the output voltage to a desired value. The control circuit turns the active switch(es) on and off, and adjusts a duty ratio to keep the output near the desired value. A duty ratio may be defined as the fraction of time that the switch is on in a single switching period. Each switching period may include a period when the switches are on, or in other words conducting current, subsequently followed by a period when the switches are off.
A reset circuit included in the forward power converter allows the magnetic flux of the transformer to reset (that is, to return to a much lower value) when the active switches are off. Resetting the magnetic flux of the transformer prevents excess stored energy from saturating the transformer (which alters properties of the transformer). The reset is generally achieved by applying a reset voltage of appropriate magnitude and duration to the primary winding when the active switches are off.
Typically, in a forward power converter, the duty ratio of the active switches are limited to 50% due to the design restrictions of the forward converter topology. Thus, when designing a forward power converter, considerations for the input voltage range of operation may be taken such that when operation is on the lower end of the input voltage range the duty ratio is close to 50%. Therefore, when the power supply receives an input voltage on higher end of the input voltage range, the duty ratio is substantially smaller (i.e., 20-30%). However, the design must also consider limiting the minimum duty ratio that the switches can operate at, to maintain accurate regulation and lower rms currents in the power switch(es).
A common technique used with the forward power converters is to extend the duty ratio beyond 50% to allow the forward power converter to operate over a wider range of input voltages and/or increase efficiency of the power converter by maximizing the duty ratio for nominal output power. A common way to implement this technique, is to use a reset circuit to apply a voltage across the primary winding when the transformer is resetting that is artificially higher in value than the input voltage that appears on the primary winding when the switches are on. However, the power converter may incur increased power dissipation due to the reset circuit.